1. Field of the Invention
This invention relates to a sheet conveying apparatus capable of continuously conveying sheets substantially without gaps, and is applied to an image forming apparatus such as a printer, a facsimile apparatus or a copier.
2. Related Background Art
As image forming apparatuses of this kind, there are, for example, copiers, printers, facsimile apparatuses, etc.
In these image forming apparatuses, there is widely used an apparatus for fixing an unfixed toner image formed and borne on a recording material (a paper leaf member such as a transfer material, photosensitive paper or electrostatic recording paper) correspondingly to desired image information by a transfer method (indirect method) or a direct method by the use of a suitable image making process mechanism of the electrophotographic type, the electrostatic recording type or the like.
Also, use is made of an apparatus of the ink jet type or the like for directly effecting image formation on a recording material by liquid including a dye or a pigment.
As a sheet feeding device for a recording material (hereinafter referred to as the sheet) in such an apparatus, there is a sheet feeding device of the automatic feeding type for selecting a sheet feeding cassette fit for the required kind or size of paper from among a plurality of sheet feeding cassettes containing sheets therein, and automatically supplying the sheets one by one from that sheet feeding cassette with the operation of forming an image.
In a sheet conveying apparatus provided with such a sheet feeding device, detecting means as described below are widely used as means for detecting the leading and trailing ends of the sheet and judging bad conveyance.
A. Lever Type
As shown in FIG. 4A of the accompanying drawings, the leading end of a sheet conveyed along guides pushes down a lever as shown in FIG. 4B of the accompanying drawings, and a flag portion on the opposite side to the center of rotation interrupts a photointerrupter to thereby detect the leading end of the sheet.
The lever portion is clockwisely biased by a spring or gravity to such a degree that the sheet is not buckled.
B. Transmission Type Sensor
This comprises a light emitting element emitting infrared light or the like toward a sheet and a light receiving element disposed in opposed relationship therewith, and detects the presence or absence of the sheet between the two by the quantity of transmitted light.
By the use of such detecting means, sheets are conveyed at predetermined sheet intervals for a designated number of recorded sheets to thereby detect the leading end and trailing end of each sheet, and judge whether the sheet is being appropriately conveyed, and during bad conveyance, the conveyance of the sheet has been stopped and a user has been informed of it so as to execute appropriate processing.
In the case of the prior art as described above, however, the following problems have arisen.
In recent years, digitization has been active and for example, the number of revolutions of a polygon mirror and the driving frequencies of an LED head and an ink jet head have suffered from more technical problems as they increase, and the rise of the cost for the solution thereof occurs.
Consequently, even in apparatuses having the same image forming speed, it is desired to shorten the spacing between sheets as much as possible to thereby increase productivity.
Also, in terms of both energy and the durability of all moved or rotated parts, it is desirable to decrease the image forming speed as much as possible.
However, when the spacing between sheets is made zero or less or approximate thereto, the leading and trailing ends of the sheet becomes undetectable by the aforedescribed detecting means, and the detection of bad conveyance for sheet conveyance becomes impossible.
That is, in the aforedescribed lever type under item A, unless the trailing end of the sheet passes the tip end of the pushed-down lever, the lever does not return to its original position and therefore, at least the spacing between sheets substantially corresponding to the length of the lever is necessary (La), and further the time required for the lever to return to its original position becomes 10 msec.
This time results in greater loss of the spacing between sheets as the sheet conveying speed becomes higher, and when for example, the time for the lever to return to its original position is 30 msec. and the sheet conveying speed is 500 mm/S, a sheet conveyance spacing of 15 mm becomes further necessary.
C. In the transmission type sensor, the loss may be smaller than in the lever type, but the spacing of about 5 mm between sheets is necessary.
Also, in the case of the transmission type sensor, detection has sometimes been impossible for sheets such as OHP having high transmissivity.
Accordingly, when the spacing between sheets is made as minute as the leading and trailing ends of the sheet cannot be detected by the aforedescribed detecting means or zero or less, the detection of bad conveyance for sheet conveyance becomes impossible.
That is, by shortening the spacing between sheets, it becomes possible to increase productivity, but by the spacing between sheets becoming short, it becomes impossible to detect the trailing ends of the other sheets than the last sheet and the leading ends of the second and subsequent sheets by the sheet detecting means and as the result, in some cases, bad conveyance cannot be detected until the recording of a designated number of sheets is finished, and in such cases, there has also been the undesirable possibility that a large quantity sheets stagnate in the bad conveying portion to thereby provide badly recorded sheets and impart damage to the machine.
The present invention has been made in order to solve the above-noted problems peculiar to the prior art and the object thereof is to provide a sheet conveying apparatus and an image forming apparatus excellent in reliability which maintain high productivity and yet can quickly detect bad conveyance when it occurs.
To achieve the above object, according to the present invention, in a sheet conveying apparatus capable of continuously conveying sheets substantially without gaps, after a predetermined number of sheets have been conveyed, the next sheet is conveyed after a predetermined spacing is forcibly provided.
Accordingly, the predetermined spacing is forcibly provided and therefore, the detection of the passage of the leading ends or the trailing ends of the sheets with a predetermined number of sheets as a unit becomes possible.
When the designated number of sheets to be conveyed is greater than the aforementioned predetermined number of sheets, the operation of conveying the next sheet after the predetermined spacing has been provided may be repeated each time the predetermined number of sheets are conveyed.
Provision may be made of detecting means for detecting the passage of the leading end of the first sheet and the passage of the trailing end of the last sheet when a plurality of sheets are continuously conveyed substantially without gaps, and the bad conveyance of the sheets may be detected from the result of the detection by the detecting means.
The aforedescribed detecting means includes first detecting means for detecting the passage of the sheets on the upstream side with respect to the direction of conveyance, and second detecting means for detecting the passage of the sheets on the downstream side of the detecting position by the first detecting means, and the spacing between the detecting position by the first detecting means and the detecting position by the second detecting means may be greater than the distance from the leading end of the first sheet to the trailing end of the last sheet when the predetermined number of sheets are continuously conveyed substantially without gaps.
Thereby, the state of bad conveyance can be known.
When the spacing between the detecting position by the first detecting means and the detecting position by the second detecting means is defined as Lp and the length of the conveyed sheet in the direction of conveyance is defined as LS, if the aforementioned predetermined number is N, N may be set so as to satisfy
Lp greater than Lsxc3x97N.
Also, N may be set so as to satisfy
Lsxc3x97(N+1) greater than Lp greater than Lsxc3x97N.
Also, the image forming apparatus of the present invention is provided with image forming means for forming images on the sheets conveyed by the above-described sheet conveying apparatus.